After looking in the SPTHEMES.XML file found in the C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS\1033 folder, you do see there is a theme with a theme name of "none". Since there is no "default" theme in 2010. So make sure if you wanna reset it to default you know that there is no default, you need to select 'none' :)